A cellular network is characterized by having a plurality of BS each with the capacity to transmit and receive data in a range of frequencies. Unlike a system with a single transmitter where only one transmission can be sent over any given frequency, the cellular network can reuse the same radio frequency in a different area for transmitting completely different information. Unfortunately, frequency reuse has its price and there is an inevitably amount of inter-cell interference from the signal transmitted by other BS which use the very same frequency. The frequency reuse factor is the rate at which the same frequency can be used in the network, and commonly known as simply reuse K (or 1/K according to some) denoting the number of cells which cannot use the same frequencies for transmission. Typical values for a frequency reuse factor are 3, 4, 7, 9 and 12 (or ⅓, ¼, 1/7, 1/9 and 1/12 depending on notation). In the following description the first type of notation is used to denote the reuse factors.
In many cases there are N antennas located at the same base station, each antenna is directed to a different direction and by doing so they divide the cell into N sectors. This division enables implementing a cell frequency reuse as discussed above. For example: reuse 1 (K=1) implies that all the sectors associated with a base station will use the same frequency partition, while reuse 3 (K=3) implies that each sector in a 3 sectors' cell, uses a different frequency or a different partition within a frequency channel. The term “reuse K” as used hereinafter, is used to encompass frequency reuse within a base station which is part of a wireless deployment, calculated for a partition within the frequency channel which uses a power level higher than a given threshold. In a configuration where a base station is associated with three sectors, in a sector operating in accordance with reuse 3 mode, a given frequency channel will be transmitting at high power, while the other two sectors will not utilize that frequency channel at all. Operating in accordance with reuse 1 mode, allows all sectors to utilize the same frequency channel.
Similarly to inter-cell interference caused by the cell frequency reuse, an inter-sector interference is caused by the sector frequency reuse. A better understanding of the areas of interference can be achieved when considering FIG. 1A and FIG. 1B. The two types of interference areas arising in reuse 1 scenario are shown in FIG. 1A for DL. The inter-cell interference occurs at the cell margin where the two adjacent cells overlap.
A relative new method to overcome some of the above-mentioned difficulties in OFDMA systems is the FFR, i.e. only a fraction of the available sub-carriers in a frequency channel are used for data transmission in a sector. This technique, when applied to 802.16 mandatory modes, lowers the average interference per sub-carrier, however does not prevent the collisions between some of sub-carriers when used in adjacent sectors or cells.
The present invention seeks to provide a solution to the above problems by improving at least some of the following parameters: cell edge user throughput, spectral efficiency and cell coverage for both UL and DL, increased spectral efficiency and broadcast traffic support and the like.